This invention relates to an eddy-current speedometer with temperature compensation and comprising a rotating permanent magnetic element made from a magnet material with low permeability, and an eddy-current rotary element provided with an indicating device and made from an electrically conductive material. By means of the alternating magnetic field which is created by the permanent magnetic element and which is carried through a circular path when the magnetic element is rotated, the eddy-current rotary element is in turn set rotating through a given angular degree which is translated by the indicating device into a reading of the rotational speed of the magnetic element.
The permanent magnet materials with low permeability such as barium ferrite, strontium ferrite or lead ferrite have a relatively low energy product and a relatively low remanence. When such magnetic materials are employed for the permanent magnetic element in eddy-current speedometers, the usual magnetization in axial or radial direction relative to the axis of rotation of the rotating permanent magnetic element of the speedometer produces a relatively low torque in the eddy-current rotary element. Also, these materials are substantially dependent on temperature. This advantage has been the main reason for the use of these materials for eddy-current speedometers only to a limited extent. As generally known, the electrical conductivity of the material from which the eddy-current rotary element is customarily made also changes according to ambient temperature fluctuations. Consequently, it has been found necessary to compensate for these changes in the magnetic flux and these conductivity fluctuations due to variations in temperature.
In accordance with the invention, the temperature compensation has been achieved by incorporating in the magnetic element a temperature compensating material having a permeability dependent upon temperature. This material is formed into a ring or disc and is positioned on the magnetic element in a way that results in part of the magnetic flux being short-circuited. When the temperature increases, the permeability of the temperature compensating material decreases in order to keep the effective flux in the active air gap constant. This material can also compensate for reductions in the electrical conductivity of the eddy-current rotary element.
By employing these temperature compensating elements, the already relatively small effective field gradient of the aforementioned magnetic materials is reduced further. In addition, it is somewhat difficult to achieve an optimum temperature compensation of the ferrite type magnetic materials within a temperature range in the order of -20.degree. C. to +80.degree. C. because this magnet material has a very low magnetic conductivity or permeability which is approximately the magnetic conductivity of air.
The known eddy-current speedometers have always been provided with a magnet which is arranged within the eddy-current rotary element. To provide for a return path for the magnetic lines of flux, there has been usually provided in addition a ferromagnetic element around the eddy-current rotary element. This ferro-magnetic element is either stationary or rotates with the central magnet fastened to the drive shaft of the speedometer. With these prior assemblies, which are characterized by relatively high manufacturing costs, the driving torque produced by the eddy-current rotary element has not been satisfactory. Contrary to the methods usually applied heretofore by those skilled in the art, the present invention is substantially different from common practice in this field.